This answer: How to check if a fence or cable has electricity without having special tools? gives some unsafe advice for checking to see if a power cable is energized or not. In particular:
On that thought, I do recall spitting on an electric wire having some effect. If you get water on the wire, it may sizzle or make some other sound if there's a significant enough of a current flowing through it, but only if there is a significant amount of current.
a COMPASS which you could use to check for an electrical current. Hold the compass near the wire, the magnetic field will move the needle if there is a current in the wire.
While the compass technique may be able to detect current flowing through a wire under some conditions (it generally only works with DC current, the magnetic field from AC fluctuates so rapidly that the needle can't keep up), there's no reason to believe that spitting on the wire can do so. And current flow is not the same as danger, if there's a metal object sticking out of an electrical outlet, there will be no current flowing through it, but if you grab it, you can still be electrocuted because there will be current flowing through you. A downed cable in the wilderness is likely to be broken, but one (or both) of the broken ends may still have a lethal voltage potential with no current flowing at all - something has to complete the circuit for current to flow, and ideally that "something" won't be a person.
If someone trusts this advice to check the safety of an electrical cable found in the wilderness, he may be in for a shock (literally) when he grabs an energized wire after his compass told him it was safe.
The question itself is a little dubious, since one shouldn't trust random wires/cables to support your weight when climbing up or down a cliff, but I suppose in some survival situations it may be necessary. The safety (or lack thereof) of using a found cable is mentioned in the answer I linked to, so I'm really just questioning the validity of his methods to test the cable to see if it's live.